The invention is directed to improvements in fuel injection systems for internal combustion engines.
In a known fuel injection system of this type (German Offenlegungsschrift 29 42 010), the relief conduit includes a control throttle the cross section of which is variable via a control motor and which in combination with a magnetic valve disposed in a parallel relief conduit and by triggering with the electronic control unit determines the injection principle. While the onset and end of fuel supply are determinable by the magnetic valve, a prolongation of the injection duration and hence quiet operation of the engine ar attainable by means of the control throttle, which can throttle down to a zero fuel quantity. By allowing part of the fuel to drain away during the injection event, this fuel quantity is compensated for by a corresponding prolongation of the injection duration.
For the rpm regulation, the engine rpm serves as a control variable, here, because an actual fuel injection quantity metering does not take place. Regulation of this kind has the disadvantage, however, that it is relatively sluggish, since the injection quantity cannot be varied except via a comparison of the actual and set-point values of the rpm inside the electronic control unit and naturally other engine parameters such as load, temperature and so forth must be processed in the electronic control unit as well. An indirect measurement of this kind is particularly disadvantageous whenever the control throttle has only a specialized task such a prolonging the injection duration and hence complicates the overall regulation considerably.
In this known injection system, the fuel return quantity can also be derived from the respective control variable of the control motor that actuates the variable throttle. However, such a compensation calculation performed in the electronic control unit on the basis of control motor feedback variables involves complex mathematical functions, since the control path of the control motor has a correspondingly complicated relationship with the relief quantity flowing through the throttle. These are higher order functions, since the change in throttle cross section and the pumping course of the injection pump cause changes in the fuel pressure in the relief conduit.